1. Field of the Invention
This invention relates to a process for preparing uniform, agglomerate-free, submicron/nanosize ceramic powders for use as starting powders for high technology ceramics. In particular, this invention relates to a process for preparing uniform, agglomerate-free, submicron/nanosize ceramic powders from a precursor solution incorporated into a polymeric foam.
2. Description of the Prior Art
The usefulness of many high technology ceramics depends upon the characteristics of the ceramic powders used as starting powders which are sintered to produce a ceramic product. In general, to achieve desirable characteristics in the finished products, a ceramic powder should consist of particles with a narrow size distribution in the submicron range. In addition, to avoid pores larger than the primary particle size, the particles should be discreet, rather than attached together in agglomerated clusters. Agglomerated clusters often produce low-density green ceramics and leave numerous large pores after sintering. Finally, it is important that the ceramic powder be free of contaminants to insure purity of the resulting high technology ceramic.
One known method of preparing powder formulations used in high technology ceramics involves the calcination of a mechanically ground mixture of metal oxides and/or carbonates in definite proportions. The milling and grinding introduces contaminants from abrasive materials which have a detrimental effect on the desirable properties and introduce a variance into each batch of powder prepared. In addition, the mechanically ground mixture requires prolonged calcination at high temperatures which promotes crystallite coarsening, an undesirable consequence in the fabrication of dense fine grain ceramics. U.S. Pat. No. 3,330,697 teaches a process for preparing lead and alkaline earth titanates and niobates from resin intermediates made from alpha-hydroxycarboxylic acids, such as citric acid, in which a hydrated oxide or alkoxide and an alphahydroxycarboxylate of titanium, niobium, and zirconium is mixed with citric acid in a polyhydroxy alcohol which is liquid below about 100.degree. C., dissolving therein at least one basic one metal compound from the group of oxide, hydroxide, carbonate and alkoxide of lead and the alkaline earth metals, and calcining the composition to remove the organic constituents. The resulting product includes agglomerated particles which require grinding after calcination. In addition, particle size is very difficult to control by this method.
Similarly, Chick, L. A. et al., "Synthesis of Air-Sinterable Lanthanum Chromite Powders", Proceedings of the First International Symposium on Solid Oxide Fuel Cells, vol. 89-11, pgs. 171-187, teaches a process for synthesizing lanthanum chromites in which metal nitrates and glycine or some other low molecular weight amino acid are dissolved in water and the resulting solution is boiled down until it thickens and ignites, producing ash that contains the oxide product. Thereafter, the oxide product is calcined, sonicated and dry pressed. This process too produces agglomerates which require grinding after calcination, thereby introducing contaminants into the ceramic powder. In addition, particle size is very difficult to control.
Numerous methods for preparing porous ceramic materials having particular physical and chemical properties are disclosed by the prior art. One such approach is disclosed by U.S. Pat. No. 2,918,392, U.S. Pat. No. 4,004,933, U.S. Pat. No. 3,907,579, U.S. Pat. No. 3,833,386, and U.S. Pat. No. 4,559,244 in which a foam or porous solid body is impregnated with a material for deposit on the surfaces of the foam or porous body and subsequently treated, for example sintered, to produce porous ceramic or ceramic coated materials. U.S. Pat. No. 3,649,354 teaches a method for producing electrically operated devices in which a liquid electrically insulating filler material, such as polyurethane, is applied to a layer of electrically active grains and allowed to contract, thus exposing the peaks of the grains, after which it is allowed to harden. U.S. Pat. No. 4,572,843 teaches a method for producing a capacitor in which an insulating composition, such as an organic polymeric compound containing a metal powder or an organometallic compound as a metal source is formed on a dielectric layer formed on an electrode, the insulating composition being heated to form a second conductive electrode.
With a somewhat different approach, U.S. Pat. No. 3,497,455 teaches a method for producing foam metallic oxides in which an aqueous solution of a metal salt (nitrate) is mixed with a frothing agent to form a noncollapsing foam, and subsequently heated up to about 3000.degree. F. to form the porous product.
Numerous other methods for preparing ceramic structures from ceramic powders are also disclosed by the prior art. U.S. Pat. No. 4,957,673 teaches a method for producing unitary layered ceramic structures having cosintered layers for use in fuel cells, such as tapes having a center layer of yttria-stabilized zirconia sandwiched between outer layers of strontium doped lanthanum manganite. U.S. Pat. No. 2,108,995 teaches an anode of film forming material and a cooperating cathode spaced by a sheet of a flexible nonfibrous albuminous sheet material which has been impregnated and made electrically conductive by the addition of a conductive electrolyte, such as ethylene glycol and citric acid. Similarly, U.S. Pat. No. 2,158,981 teaches an electrolytic condenser having a highly viscous or pasty electrolyte where the electrolyte is, for example, citric acid and ethylene glycol.
U.S. Pat. No. 3,180,741 teaches a method for producing solid products from liquid polymers with polyvalent metallic salts using a mono- or polycarboxylic acid. U.S. Pat. No. 3,386,856 teaches a method for manufacturing a device consisting mainly of oxidic dielectric material in which the device is provided with electrodes and at least one of the surfaces of the device on which the electrodes are provided is superficially oxidized until an insulating junction layer is formed.
U.S. Pat. No. 3,427,195 teaches a process for producing an electrolytic condenser in which a metal foil is coated with a liquid film of water soluble nitrates or oxalates and finally suspended particles of a water insoluble refractory compound and heated to produce an electrically insulating refractory oxide which, together with a refractory compound, forms the separator coating on the metal foil.
Various methods for preparing dielectric ceramic powders are also taught by the prior art including U.S. Pat. No. 3,647,364 which teaches a process for preparing high purity, submicron, dielectric ceramic powders using alcoholates; U.S. Pat. No. 3,965,046 which teaches a process for making metal bearing powders from organometallic salt seeds; U.S. Pat. No. 4,004,917 which teaches a process for producing acicular metallic powders from organometallic salts by precipitation or growth of the salt in the presence of complexing agent; similarly, U.S. Pat. No. 4,146,504 which teaches a process for producing structures formed from powders of high porosity made using organometallic salts and glycol; U.S. Pat. No. 4,751,070 which teaches a method for synthesizing submicron particles of ceramic or metallic materials at very low temperatures in which a nitrate source is combined with an inorganic reducing fuel to provide a chemical precursor for the particular ceramic or metallic material, which precursor is exothermically decomposed in a controlled atmosphere at a temperature of about 200.degree. C. below the endothermic decomposition temperature of the nitrate source; U.S. Pat. No. 4,757,037 in which a suspension formed from a mixture of a solution of titania containing elementary crystallites of titanium oxide and a solution of neodymium nitrate or a solution of barium and neodymium nitrates is dried to obtain a dried product and calcined at a temperature of 800.degree. to 1300.degree. C. to obtain an ultrafine dielectric powder; U.S. Pat. No. 4,800,051 which teaches a method for ceramic fabrication involving hydrolyzing a suitable metal alkoxide to form a slurry, drying the metal oxide powder in the slurry, granulating and calcining the metal oxide powder, ballmilling the calcined metal oxide powder as a slurry to maximize powder dispersion in the solution, compacting the dispersed powder from the ballmilled slurry into a powder compact, drying the powder compact and sintering the powder compact at a suitable relatively low sintering temperature; U.S. Pat. No. 4,845,056 in which a solution of ceramic oxides or hydrous oxides is continuously pressurized and heated to in excess of the critical temperature and pressure of the solution solvent, transforming the solvent to a gas and subsequently separating it from the fine particulate ceramic oxide powder; and U.S. Pat. No. 4,141,763 in which a stream of an aqueous solution of metal salt and a solution containing a reducing material are injected into a uniformly applied magnetic field from nozzles and immediately mixed as they impinge on one another in the form of sprays to cause a reaction between them. However, none of the prior art teaches a method for preparing uniform, agglomerate free submicron/nanosize ceramic powders by incorporating a precursor solution within a polymeric foam.